1. Field of the Invention
This invention relates to an analog-to-digital converter, and more particularly, to a compact, high speed analog-to-digital converter.
2. Description of the Prior Art
Prior art high speed analog-to-digital (A/D) converters can generally be classified as either parallel-type converters or serial-type converters. Parallel type A/D converters which convert an analog input signal to, for example, 8-bit digital output signals generally require 255 complex voltage comparing circuits connected in parallel. These circuits require a large number of devices and large power consumption and result in a large chip size for the IC (Integrated Circuit). Serial-to-parallel A/D converters having an output of m upper bits and n lower bits which convert an analog input signal to 8-bit digital output signals require 30 complex voltage comparing circuits, result in low power consumption, and result in a small size chip for the IC. However this serial-to-parallel A/D converter requires a digital-to-analog converter and subtracter for A/D converting with upper bits and lower bits separately, so that it is difficult to convert the analog input signal to digital output signal exactly. It is feared, for example, that the serial-to-parallel A/D converter produces non-sequential values for converting a continuously increasing analog input signal to digital signal.